This invention relates to dashpots or dampers in which the damping action is provided by the displacement of liquid in a liquid-filled chamber.
The dashpot may for example be a semi-rotary type, in which at least one vane in the chamber is pivotable relative to the chamber to displace the liquid, or a linear type in which the chamber is cylindrical and a piston is relatively axially displaceable within it.
In British Patent No. 1566699, it is explained how the application of torque to a semi-rotary dashpot incorporating a vaned rotor produces a rise of fluid pressure in the space on one side of the vane and a decrease in pressure on the other side and how this can lead to an excessive pressure reduction in part of the liquid which causes voids to appear in the liquid, leading to problems from the degassing of the liquid.
That earlier patent describes an arrangement for limiting the pressure drop by providing communication through the rotor to an inner space surrounding a shaft on which the rotor pivots. In that space a compensation chamber sealed from the exterior by a flexible diaphragm connected between the casing and the rotor, the diaphragm forming a boundary subject to atmospheric pressure. Because the diaphragm is able to flex under atmospheric pressure, additional fluid is able to flow into the dashpot chamber to limit a sub-atmospheric pressure drop on either side of the vane, so avoiding the problems explained.
The arrangement disclosed in that earlier patent is rather complex and presents assembly difficulties, both of which factors increase the production costs. It is possible by use of the present invention to provide a simpler solution to the problem.